Add a Windows and macOS smoke job to CI

[Co-Messi]

Body:

CI currently runs on Linux. Because actwhy shells out to git and does path handling, a lightweight smoke job on windows-latest and macos-latest would catch platform-specific breakage (path separators, line endings, git availability) early.

Scope:

• Add a matrix leg (or a separate job) for windows-latest and macos-latest.
• Run npm run build and a minimal actwhy invocation against a fixture repo.
• Keep it fast — this is a smoke test, not the full suite.